Stars Coco Gauff and Taylor Fritz put a halt to China's stirring United Cup campaign with victories in their singles matches on Wednesday as top seeds United States powered through to the semifinals.

The United States joined Kazakhstan in the final four, while the group stage was also completed on day six with Poland qualifying for the quarterfinals after defeating Czechia 2-1 while host Australia overcame Britain 2-1.

But Britain and Czechia did enough to qualify for the knockout stages.

After finishing as the highest placed group runner-up in Perth, China's chances of a quarterfinal upset were dealt a blow when in-form Gao Xinyu was forced to stand down due to injury.

Zhang Shuai of China serves during the women's singles match against Coco Gauff of the United States in the quarterfinal between China and the United States at the 2025 United Cup tennis tournament in Perth, Australia, Jan. 1, 2025. (Photo by Zhou Dan/Xinhua)

The 175th ranked Gao had brilliantly stepped up in place of world number five Zheng Qinwen to beat Beatriz Haddad Maia and Laura Siegemund in breakout performances.

Zhang Shuai, ranked 205, put in a gallant performance against Gauff at Perth's RAC Arena and broke her three times in the opening set.

But the world number three responded to the challenge and stepped up her firepower along the baseline to eventually prevail 7-6 (4), 6-2 in one hour and 34 minutes.

"I felt like I couldn't get her on defense," said Gauff, the 2023 US Open champion. "She was playing so aggressive on me and I felt like I had to go more on my second serves. I had to take my chances."

World number four Fritz sealed a semifinal spot for the Americans with a commanding 6-4, 6-4 victory over Zhang Zhizhen in one hour and 20 minutes. The US Open finalist served strongly and capitalised on an error-strewn performance from Zhang, who finished with a 1-2 record in the tournament.

Taylor Fritz (R) of the United States greets Zhang Zhizhen of China after their men's singles match in the quarterfinal between China and the United States at the 2025 United Cup tennis tournament in Perth, Australia, Jan. 1, 2025. (Xinhua/Ma Ping)

Earlier in Perth, Kazakhstan took advantage of world number two Alexander Zverev's withdrawal to end Germany's title defense. Zverev, a two-time grand slam finalist, pulled out due to a biceps strain in a concern ahead of the Australian Open.

The 254th ranked Daniel Masur had the tough task of replacing Zverev and put up a fight, but Alexander Shevchenko rallied to secure Kazakhstan's spot in the final four with a 6-7 (5), 6-2, 6-2 victory at a very hot RAC Arena.

World No. 6 Elena Rybakina had earlier put Kazakhstan ahead after a commanding 6-3, 6-1 victory over Siegemund to kick-start the knockout round.

It was the 2022 Wimbledon champion's third consecutive straight sets win after beating Jessica Bouzas Maneiro and Maria Sakkari earlier in the tournament.

"Feeling better on the court and physically also," said Rybakina, a finalist at the 2023 Australian Open. "It was a long trip. It's not easy at the beginning, but really happy with the performance and really looking forward to the next match."

Earlier in Sydney, world No. 2 Iga Swiatek continued her strong comeback with a comprehensive 6-3, 6-4 victory over former French Open finalist Karolia Muchova.

After Hubert Hurkacz's three-set loss to Tomas Machac in the men's singles, Swiatek needed a victory to keep alive Poland's hopes of automatically qualifying for the quarter-finals.

There had been intrigue ahead of the tournament over how the five-time grand slam champion would fare after a one-month suspension for testing positive to the banned substance trimetazidine.

But Swiatek has looked sharp and backed up her crushing opening victory over Norwegian Malene Helgo by fending off the wily 22nd ranked Muchova, who had troubled her in previous matches.

"Honestly, I love playing against Karolina because she always brings precision and intensity and a little bit of a different game than most of the girls," Swiatek said.

"In tough moments I just tried to loosen up my hand and force myself to not change any movements because of any thoughts."

Swiatek was Poland's driving force in the deciding mixed doubles as she combined with Hurkacz to beat Muchova and Machac 7-6 (3), 6-3 and seal top place of Group B.

Czechia advanced as the highest placed runner-up in Sydney and they will face Italy in the quarterfinals.

In Sydney's night session, Katie Boulter silenced the partisan home fans with a convincing 6-2, 6-1 win over Olivia Gadecki in the women's singles. But Alex de Minaur, Boulter's fiance, levelled the tie with a 6-2, 6-1 victory over Billy Harris in 76 minutes.

"I'm very happy with the win today," de Minaur said. "It was against a tough opponent. I had to play some very solid tennis."

Even though Australia's chances of progression had ended, de Minaur and Gadecki beat Olivia Nicholls and Charles Broom in a straight sets consolation victory in the mixed doubles.

Britain will play Poland in the quarterfinals on Thursday.